Traces of anthrax have been discovered in the US state department and CIA headquarters, it emerged today.

Mail rooms at the two buildings are shut and a postal worker at the state department has tested positive for inhalation anthrax, the most dangerous form of the disease.

The state department spokesman, Richard Boucher, said that it was not known how the employee had been exposed, but that he was in hospital for treatment.

The 59-year-old reportedly told doctors that he "never" went to the Brentwood sorting office in Washington where other postal workers contracted the disease.

A second man who works at the same mail facility as the infected worker has flu-like symptoms and is being tested at a hospital.

CIA officials said the level of contamination at the agency's headquarters in Langley, Virginia, was "medically insignificant" and no employees have contracted the disease.

In New York, anthrax was detected on four mail-sorting machines at a processing station that handles millions of parcels a day.

The US post office has now set up spot checks at its facilities nationwide as the bioterrorist scare spreads across America.

The latest cases as the US homeland security director, Tom Ridge, disclosed that the anthrax addressed to the senate majority leader, Tom Daschle, had been altered to make it more deadly.

"It is highly concentrated. It is pure and the spores are smaller," he said. "Therefore they're more dangerous because they can be more easily absorbed in a person's respiratory system."

Mr Ridge identified the disease as the Ames strain - used in American biological warfare research and vaccine testing - but said that he did not know who was responsible for the attacks.

Since the first outbreak three weeks ago, there have been 13 confirmed anthrax cases - seven cases of inhalation and six of the less dangerous skin form of the disease. Most are linked to anthrax-spiked mail that has passed through New Jersey, New York or Washington.

Anthrax has also been found in Florida, where one man died, although authorities have not yet found tainted mail there.